Add another canister filter.
Then add more surface movement than what you had, a slight rippling is good.

Now add more CO2.

You will lose some CO2 this way, but it's easy to add more.
Larger fish require more/higher O2.

Good plant growth will help.
Do not add CO2 at night.

Do this slowly, do not try and wing it and add lot so of CO2 or then none at all, slowly raise it up and note fish health/color.

I never had these issues, I've seen them in other folk's tanks though........
Once they start eating and coming out more, then the CO2 can be upped some also.

Just do not push too fast.

Regards,
Tom Barr